A system of this type is disclosed in German 31 07 780. In particular, it is referred to the embodiments shown in FIGS. 2 and 3 as well as the specification relating thereto. Accordingly, a hydrostatic machine having an adjustable volume acting as a secondary unit is connected to a supply line in which a constant or, respectively, an impressed pressure is maintained which secondary unit is coupled to a signal generator supplying a fluid flow which volume is proportional to the speed of the secondary unit, i.e. said fluid flow corresponding to an actual value of the speed. A desired value of the speed is defined by a control flow which volume is adjusted by a valve. The position of the valve also determines the direction of rotation of the secondary unit. From both said control flows one defining the actual speed value and the other the desired speed value, a differential flow is produced passing through a throttle, wherein an adjusting means of the secondary unit is controlled by the pressure difference across the throttle to adjust the through-put volume of the secondary unit such that a desired speed is obtained. The document referred to discloses a directly controlled adjusting system which, of course, can be replaced by a pilot controlled system. To accomplish this, the pressure difference across the throttle which is proportional to the differential flow or, respectively, the control error is supplied to the fluid chambers of a pilot controlled valve, whereas the fluid for controlling the adjusting piston of the secondary unit is taken from the supply line of constant or impressed pressure. Furthermore, U.S. Pat. No. 4,920,747 discloses a mechanical feedback of the pivot angle of the secondary unit to the adjusting valve thus obtaining a feedback loop with respect to force or, respectively, distance.
However, due to the differential volume flow across the throttle the load stiffness of the known secondary control means is rather poor, i.e. the control reacts to load variations by substantially varying the speed.
It is thus an object of the present invention to take measures for increasing the load stiffness while maintaining or even improving the stability and sensitivity of the control means.